Method and device for diagnosing a reading

ABSTRACT

A method for diagnosing a reading ( 10, 12 ) or the sensor ( 106, 110 ) delivering the reading, whereby the sensor ( 106, 110 ) is spatially and/or functionally associated with an exhaust gas catalytic converter unit ( 108 ) which is arranged after a combustion engine ( 100 ) and a device functioning according to the method are provided, whereby a first reading ( 10 ) from a first sensor ( 106 ), arranged before the exhaust gas catalytic converter unit ( 108 ) in the flow direction of the exhaust gas and a second reading ( 12 ) from a second sensor ( 110 ), arranged after the exhaust gas catalytic converter unit ( 108 ) in the flow direction of the exhaust gas, is processed, whereby a special operating state of the combustion engine ( 100 ) is awaited or induced and, upon reaching the special operating state, it is monitored whether the first and second reading ( 10, 12 ) fall below a first threshold value ( 16 ) within a predetermined or predeterminable first time span ( 18 ).

The invention relates to a method for diagnosing a reading or the sensor delivering the reading, in particular a reading in the exhaust gas branch of a combustion engine, and a device functioning according to the method. The reading inside or outside the exhaust gas branch of a combustion engine is preferably a reading indicating the oxygen quantity or concentration in the exhaust gas. The related sensor is correspondingly referred to as an oxygen sensor. The invention can advantageously be used for diagnosing an exhaust gas catalytic converter unit arranged after a combustion engine.

Methods and devices for diagnosing readings or the relative sensor are known in various forms. Often in these cases, for example, a signal path is implemented redundantly, whereby at least a further sensor is associated with the sensor which is actually to be diagnosed and it is monitored whether both sensors deliver plausible signals.

Methods and devices for diagnosing an exhaust gas catalytic converter unit which is arranged after an internal combustion engine are known in various forms. The use of a sensor respectively at the entrance and the exit of the exhaust gas catalytic converter unit is also known, for example from U.S. Pat. No. 5,417,061, whereby the time interval between the points at which the two sensor signals alter their polarity is calculated. Further comparable solutions are known from U.S. Pat. No. 5,319,921 or U.S. Pat. No. 6,338,243, whereby diagnosis takes place in each case with a saturated air-fuel mixture. A method is known from U.S. Pat. No. 6,804,951, whereby a time offset is detected between the signals of the two sensors and the detected time offset is compared with a threshold value in order to diagnose the exhaust gas catalytic converter unit. A method is known from U.S. Pat. No. 5,771,685 which is based on detecting a time lag between a first and a second point in time. Here, these two points in time relate, in the case of each sensor, to a transition from a reading which characterises a lean air-fuel ratio to a reading which characterises a saturated air-fuel ratio. In U.S. Pat. No. 5,743,086 and U.S. Pat. No. 5,737,917 the relationship between the signal lengths of the two sensors is used to diagnose the exhaust gas catalytic converter unit. Finally, a method is known from U.S. Pat. No. 5,140,810, whereby, in a specific operating state of the combustion engine, it is monitored whether a first reading for an exhaust gas composition falls below a specific threshold value and whether a second reading for the exhaust gas composition also falls below the specific threshold value, whereby there is assumed to be an error situation if both readings lie below the specific threshold value.

The invention is based on the problem of providing a further method and a device, functioning according to the method, for diagnosing a reading or the sensor delivering the reading and, in a special embodiment of the invention, a method and a device, functioning according to the method, for diagnosing an exhaust gas catalytic converter unit.

According to the invention, a method is provided for diagnosing a reading or the sensor delivering the reading, which sensor is spatially associated with an exhaust gas catalytic converter unit (catalytic converter) arranged after a combustion engine. A first reading is obtained from a first sensor arranged before the exhaust gas catalytic converter unit in the flow direction of the exhaust gas (exhaust gas flow direction). A second reading is obtained from a second sensor arranged after the exhaust gas catalytic converter unit in the flow direction of the exhaust gas is processed. In accordance with this invention, a special operating state of the combustion engine is awaited or induced and, when the special operating state is reached, it is monitored whether the first and the second readings fall below a first threshold value within a predetermined or predeterminable first time span. The two sensors are, in particular, oxygen sensors, so that the two readings provide a measurement of the oxygen concentration in the exhaust gas in the case of oxygen sensors.

By monitoring whether both readings fall below a predetermined or predeterminable first threshold value, a first piece of information regarding the plausibility of the two readings can be obtained. The induced special operating state namely causes a change in the air-fuel ratio or involves such a change. The altering of the air-fuel ratio also changes the composition of the exhaust gas, such that it can be expected that, after a certain period of time after the inducement of the special operating state, this can be seen from the or both readings. This information can also be utilised as an indicator regarding the state of the catalytic converter.

A further piece of information regarding the plausibility of the two readings can be obtained by further monitoring whether both readings fall below the first threshold value within a predetermined or predeterminable first time span. Due to the fact that one of the sensors is arranged before the catalytic converter unit and the other sensor is arranged after the catalytic converter unit in the exhaust gas flow direction, the catalytic converter unit acts like a delay element. In the case of oxygen sensors, in the special operating state, the oxygen concentration in the exhaust gas changes more rapidly in front of the catalytic converter unit than behind the catalytic converter unit in the exhaust gas flow direction, which is, in particular, due to the capability of the catalytic converter unit to store oxygen. The duration, up to which the alteration of the oxygen concentration in the exhaust gas is also measurable after the catalytic converter unit, is thus substantially defined by the catalytic converter unit, such that a length of time which reflects this can be predetermined by the second time span. This information can in turn collectively be utilised as an indicator regarding the state of the catalytic converter. A combination of both pieces of information can be used advantageously in order to diagnose the state of the catalytic converter.

The special operating state of the combustion engine is, advantageously, a state in which no fuel is supplied to the combustion engine, i.e. in which the combustion engine pumps virtually only the aspirated air and expels it again as exhaust gas. In this operating state, the oxygen concentration in the exhaust gas is particularly high, due to the fact that, without fuel, combustion, in which the aspirated oxygen is used up, does not take place in the combustion engine.

Advantageously, if both readings fall below the first threshold value within the first time span, it is further monitored whether both readings remain below the first threshold value for the duration of a predetermined or predeterminable second time span. In this manner, it is ensured that not merely an instantaneous reading, which could indeed possibly have been erroneous, is used for the diagnosis, but that the first threshold value is not exceeded for a definite period, namely the second time span.

Advantageously also, a counter is incremented or decremented if one of the two readings exceeds the first threshold value within the duration of the second time span. In this manner, it can more or less be logged how often the first threshold value is exceeded. Here, the counter can only relate to the duration of the second time span, such that it is thus only logged how often one of the two readings exceeds the first threshold value within the duration of the second time span. The counter would, additionally, be reset upon each new performance of the method. Precisely in this manner, it can also be provided that, upon each new performance of the method, the last valid counter status is used, such that it is logged how often one of the two readings is exceeded in total, i.e. even in the case of several performances of the method.

The current status of the counter is preferably monitored with regard to the reaching or the exceeding of a first counter threshold value, by at least one counter threshold value being associated with each counter and by a signal being generated if the counter threshold value is reached or exceeded. If the signal is activated, the counter threshold value has been reached or exceeded, such that the signal indicates a state in which at least one of the two readings is not plausible. This is due, then, to the signal path, to one of the sensors or to the exhaust gas catalytic converter unit.

A specific feature then exists if a marked deviation remains between both readings during the second time span, i.e. a deviation above a predetermined or predeterminable further threshold value relating thereto. This deviation can then be added to one of the two readings as an offset, such that, as a result, the deviation disappears or at least moves closer to zero.

In the use, according to the invention, of the method for diagnosing an exhaust gas catalytic converter unit, the signal can be directed, for example, to a display apparatus, for example, to a control light in the dashboard, and, in this manner, be used for optically signalling a specific state of the catalytic converter. If the signal is activated, the counter threshold value has been reached or exceeded, such that the signal indicates a suboptimal state of the catalytic converter. Several counter threshold values can also be used, whereby, upon reaching a first such counter threshold value, there is, as yet, no indication on the dashboard and whereby, upon reaching further threshold values arranged subsequently, a respective display of a first, second, etc. “ageing state” of the catalytic converter is indicated.

The invention relates in the same manner to a device which functions according to the method as described above. To this end, the device has means, for example, a processing unit, in particular a microprocessor or a microcontroller for carrying out the method. Here, the processing unit can be, for example, a component of the so-called engine electronics. Further, the control equipment of the microelectronics, which normally has a processing functionality of the microprocessor type or microcontroller type, comes under consideration as a processing unit. The processing unit has, for receiving each of the two readings, one input, for example in the case of so-called multiplexed inputs, or a respective input for each reading. An A/D conversion is, of course, required in a manner known per se for the electronic processing of the or each reading, which A/D conversion can be integrated into either the respective sensor or the processing unit or the signal path between the sensor and the processing unit. Using the or each received reading, the processing unit can execute a program code, by means of which the method is implemented as described above.

The invention also lastly relates to a computer program product, in particular a machine-readable storage medium for computer program data, having program code for implementing the method described above, whereby the program code is transferable into a memory of a processing unit of the type described above or a memory associated with, or which can be associated with, such a processing unit and can be processed by the processing unit and whereby, in the processing of the program code by the processing unit, the method steps of the above described method are performed.

In this case, the invention works on the assumption that a diagnosis of a reading, in particular of a reading which influences central functions of a system, for example, of a combustion engine, is a sensible way of obtaining a measurement of its plausibility. In the case of oxygen sensors, the above-mentioned first sensor, for example, is the so-called “lambda probe”, which is significant in the regulation of the air-fuel ratio. If malfunctions of the lambda probe or of the reading provided are recognised in good time, an undesired pollutant emission and/or a possible deterioration of the exhaust gas catalytic converter unit can be prevented or such a situation can be precluded by means of timely maintenance. In this case, the invention goes far beyond the known redundant implementation of sensors and sensor apparatuses including the respective signal path, due to the signals being examined in a targeted manner with regard to an “anticipated behaviour”. In this context, the invention suggests, in particular, simple method steps, by means of which the anticipated behaviour can be recognised. In general, the invention can thus also be described as follows: a method is provided for diagnosing a reading from a group of at least two readings or for diagnosing the sensor delivering the or each reading, whereby a special operating state of the system with which the or each sensor is associated—for example of the combustion engine—is awaited or induced and, upon reaching the special operating state, it is monitored whether the or each reading demonstrates an anticipated behaviour. The presence of the anticipated behaviour can, for example, be seen in that, in the case of two readings, both readings fall below a first threshold value within a predetermined or predeterminable first time span.

The invention further works on the assumption that a diagnosis of an exhaust gas catalytic converter unit is carried out in order to obtain a measurement of its potential deterioration. Reasons for a deterioration are, for example, thermal ageing and chemical or mechanical contamination. Thermal ageing results from a continuous reduction in the effective surface of the exhaust gas catalytic converter, which, for its part, is caused by sintering processes (leakage processes). Such sintering processes can be noted, above all, at high operating temperatures. If the catalytically effective layer of the exhaust gas catalytic converter is rendered unusable by chemical reactions with foreign matter, for example additives from the fuel or oil used, reference is made to a chemical contamination of the exhaust gas catalytic converter. In contrast, if the catalytically effective layer is covered and thus deactivated in places by matter from the fuel or engine oil, for example lead, sulphur or metal compounds, this is referred to as mechanical contamination. If such deterioration exists, the effectiveness of the exhaust gas catalytic converter is reduced and the undesired emission of pollutants is increased correspondingly. The recognition of the extent of potential deterioration is thus important, in order to indicate a potentially necessary replacement of the exhaust gas catalytic converter.

The advantage of the invention lies particularly in the fact that the method is easy to implement.

In the following, an embodiment of the invention will be described in greater detail using the drawings. Objects or elements corresponding to one another are given the same reference numbers in all figures.

FIG. 1 shows a schematic depiction of the exhaust gas branch of a combustion engine and

FIG. 2 shows a schematic depiction of the temporal progression of readings, as they are delivered by a first and a second sensor, which are arranged before and after the catalytic converter in the flow direction of the exhaust gas.

FIG. 1 shows a schematic depiction of an exhaust gas branch of a combustion engine 100. The combustion engine 100 includes, in a manner known per se, a number of cylinders 101. Situated respectively before and after the combustion engine 100 in a manner known per se are an intake manifold 102 and an exhaust manifold 104.

Attached to the exhaust manifold 104, or to its end, is arranged a first sensor 106, which is thus arranged before an exhaust gas catalytic converter unit 108 in the flow direction of the exhaust gas. A second sensor 110 is provided which is attached to the exhaust gas catalytic converter unit 108. In the further course of the exhaust gas branch, an exhaust silencer or the like and other elements known per se of the exhaust gas branch of a combustion engine 100, are arranged. These are not depicted.

In addition to the exhaust gas branch, engine electronics 120 are schematically depicted, which, in a manner known per se, have a processing functionality, by including a microprocessor or microcontroller or the like (not depicted) or, in an otherwise suitable manner, providing such a processing functionality. Furthermore, a memory 122 is associated with the engine electronics 120, with software, for example, being stored in said memory by means of which software specific control or regulation methods can be performed, with which the combustion engine 100, or units associated with the combustion engine 100, can be controlled, regulated and/or monitored.

A respective reading, first and second reading 10, 12, is delivered by the first and second sensor 106, 110, which readings are supplied to the engine electronics 120 at inputs provided for this purpose.

In the combustion of fuel in the combustion engine 100, for example of petrol in an otto engine, the pollutants carbon monoxide and nitrogen oxides and hydrocarbons are generated along with water and carbon dioxide, in a known manner. The pollutant emission can be reduced with the aid of the exhaust gas catalytic converter 108. In this case, the following reactions take place at the catalytic converter as a component of the exhaust gas catalytic converter unit 108: 2CO+O₂→2CO₂ 2C₆H₆+15O₂→12CO₂+6H₂O 2NO+2CO →2CO₂+N₂

The carbon monoxide is thus oxidised into carbon dioxide. The non-combusted hydrocarbons are oxidised into water vapour and into carbon dioxide and the nitrogen oxides are reduced into nitrogen. In order for these three reactions to be able to take place alongside each other with high turnover, the oxygen proportion in the exhaust gas must be electronically regulated. The regulation is effected on the basis of the reading from a so-called lambda probe arranged between the engine and the catalytic converter. The lambda probe which is known per se is an example of the above-mentioned first sensor 106. In this case, the lambda probe measures the oxygen content of the exhaust gas before the catalytic converter in the flow direction of the exhaust gas. Using the lambda probe reading, there then takes place, for example by means of the engine electronics 120, the regulation of the air-fuel mixture such that the “air ratio” (λ) differs as little as possible from an optimal value at “1.0”, by restricting, where appropriate, the air supply in the carburettor. The air ratio is defined as follows: $\lambda = \frac{{oxygen}\quad{quantity}}{{{stoichiometrically} \cdot {necessary} \cdot {oxygen}}\quad{quantity}}$

At λ<1, there is a rich mixture with too little oxygen (“insufficient air”) and at λ>1 there is a lean mixture with too much oxygen (“excess air”).

FIG. 2 shows a schematic depiction of the temporal progression of readings, as they are delivered by the first and second sensor 106, 110, functioning as oxygen sensors. The progression of the first reading 10 and the progression of the second reading 12 is depicted. Further, the progression of the air-fuel ratio 14 is depicted. Time is plotted on the x-co-ordinate. On the left of the two ordinates, the strength of the signal delivered by the two sensors 106, 110, is plotted in millivolts (mV) and on the right ordinate the air-fuel ratio is plotted.

At the point in time t1, in the case of the depicted configuration, a special operating state of the combustion engine 100 arises, which is either awaited, in order to begin the method according to the invention when such an operating state is recognised, or which is induced by a first method step of the method, in order to then carry out the further method steps in this operating state. The special operating state is preferably a state in which no fuel is supplied to the combustion engine. The operating state is thus always commenced, for example, if a driver does not actuate the accelerator pedal for a specific period when operating a motor vehicle, i.e. “takes his/her foot off the accelerator” or if a so-called “intrusive” test is carried out, in which a lean mixture is provided by the engine control but is still injected. Such an operating state can be recognised, for example, from the position of the accelerator pedal, from the quantity of the fuel supplied or also from the reading of at least one of the two sensors 106, 110, in particular from the reading of the first sensor 106.

In the present case, the first reading 10 delivered by the first sensor 106 drops rapidly at the commencement of the special operating state at the point in time t1. Here, the first reading falls below a first threshold value 16 relatively soon after the point in time t1. The second reading 12 delivered by the second sensor 110 falls only after some delay, which can be explained by the capability of the catalytic converter to store oxygen (and indeed in particular during the lean phase with the oxygen excess, which is the phase in existence in the special operating state if no fuel is supplied). The second reading 12 also drops below the first threshold value 16 after a fairly long time.

It is important for assessing the plausibility of the readings 10, 12 and/or for diagnosing the sensors 106, 110 and for recognising a properly functioning catalytic converter, that both readings 10, 12 fall below the first threshold value 16 within a predetermined or predeterminable first time span 18, whereby the predetermined or predeterminable first time span 18 is given here by the interval between two points in time t2 and t3. The first time span 18 can, of course, also begin upon commencement of the special operating state, i.e. at the point in time t1 and would then extend correspondingly from the point in time t1 to the point in time t3.

In addition to the examination of whether both readings 10, 12 fall below the first threshold value 16 within the first time span 18, the method according to the invention can be supplemented by a further analysis, which relates to the fact that it is monitored whether both readings 10, 12 remain below the first threshold value 16 during a predetermined or predeterminable second time span 20. In the representation in FIG. 2, the second time span 20 is given by the interval between the point in time t3 and a further point in time t4.

If at least one of the two readings 10, 12 does not fall below the first threshold value 16 during the first time span 18, an error situation is recognised. In the same manner, an error situation is recognised if at least one of the two readings 10, 12 exceeds the first threshold 16 during the second time span 20. In this context, error situation means that an implausible reading 10, 12 has been recognised. This, in turn, is either due to defectiveness in the reading 10, 12 itself, to an error in the respective signal path or to defectiveness in one of the sensors 106, 110 as well as ultimately to deterioration of the exhaust gas catalytic converter unit 108. In each of these two error situations, it can be provided that a signal indicating the error situation is generated which causes either the storing of an entry relating thereto in an engine electronics 120 memory 122 provided for this or the activation of an optical or acoustic notification element, for example, a control light in the dashboard.

Alternatively or additionally, it is possible that a counter can be incremented or decremented, if one of the two readings 10, 12 exceeds the first threshold value 16 within the second time span 20. Thus, the activation of a notification element of the type outlined above can then take place in a targeted manner, if a specific predetermined or predeterminable number of error situations is recognised, such that a potential signal to the driver only takes place if the number of error situations or, where applicable, also an accumulation of error situations within a particular period of time, exceeds a predetermined or predeterminable threshold value. Further, it can be provided that, in the case of such monitoring, several threshold values are taken into consideration, such that, for example, in the case of notification elements in the dashboard, the “ageing state” of his/her exhaust gas catalytic converter unit 108 can also be displayed, for example, optically to the driver. The electrical signals, by means of which such notification elements would be triggered, can, of course, also be processed by the engine electronics, in order to store all of the error situations in a memory 122 which is associated with the engine electronics 120, in particular to store them with the respective point in time of the occurrence, such that, during maintenance work on the motor vehicle, this memory 122 can be read and thus, a complete piece of information regarding potential error situations in the exhaust gas catalytic converter unit 108 is made available.

Besides being able to be used for the diagnosis of reading 10, 12 or sensor 106, 110 and the exhaust gas catalytic converter unit 108, the readings 10, 12 delivered by the first and second sensors 106, 110 can also be used for further optimisations. Basically it is the case that, in an “ideal” system, one would anticipate that, towards the end of the second time span 20, the first and the second readings 10, 12 would be approximately identical. A steady state deviation, also referred to in the following as an offset, can be treated in a different manner according to a separate aspect of the invention having its own inventive quality: firstly, according to this, one of the two readings 10, 12 can be durably acted upon by the detected offset in order to correct it. Thus if, for example, it is detected that, towards the end of the time span 20, a deviation of, for example, 50 mV remains between the two readings 10, 12, the second reading 12, for example, can be durably reduced by this offset value, i.e. 50 mV. Further, the detected offset can additionally or alternatively also be stored in the engine electronics 120 or a memory 122 which is associated with the engine electronics 120, such that the fact that such an offset remains and, where applicable, additionally also the last value of such an offset or a sequence of detected offset values, are available for maintenance operations on the motor vehicle. The maintenance personnel can then, from the data, make inferences regarding engine adjustment and, where applicable, readjust individual adjustments. A possibility for such an adjustment is the acting upon one of the two readings 10, 12, for example the second reading 12, by means of a constant summand according to the detected offset or an average value from the quantity of detected offsets. The latter option of course only then comes reasonably into consideration, if the engine electronics, as described above, do not already arrange for the automatic consideration of the offset. The storage of such data, i.e. the storage of the fact that an offset exists, and, where applicable, additionally the storage of individual or multiple offset values, can also cause the triggering of so-called “Detection Trouble Codes” (DTC; ISO 15031-6), whereby such DTC, as described above, can also be read and evaluated during maintenance work. Finally, the difference which remains at the end of the second time span 20 between the first and second reading 10, 12 can also be used in order to carry out an error correction during the regulation of the air-fuel ratio depending on the lambda probe reading, i.e. depending on the first reading 10, by returning the offset, i.e. the detected deviation between the first and second reading 10, 12 at the end of the second time span 20 as an error. The regulation would then not only adjust the required air-fuel ratio, but would also ensure that the difference between the first and second reading 10, 12 would move towards zero at the end of the second time span 20.

Thus, in short, the invention can be represented as follows:

A method for diagnosing a reading 10, 12 or the sensor 106, 110 delivering the reading, whereby the sensor 106, 110 is spatially and/or functionally associated with an exhaust gas catalytic converter unit 108 which is arranged after a combustion engine 100, and a device functioning according to the method are provided, whereby a first reading 10 from a first sensor 106, arranged before the exhaust gas catalytic converter unit 108 in the flow direction of the exhaust gas, and a second reading 12 from a second sensor 110, arranged after the exhaust gas catalytic converter unit 108 in the flow direction of the exhaust gas, is processed, whereby a special operating state of the combustion engine 100 is awaited or induced and, upon reaching the special operating state, it is monitored whether the first and second reading 10, 12 fall below a first threshold value 16 within a predetermined or predeterminable first time span 18. 

1. In a system comprising a combustion engine producing an exhaust gas, a catalytic converter for processing said exhaust gas, a first sensor arranged in the exhaust gas upstream of the catalytic converter and providing a first reading, and a second sensor arranged in the exhaust gas downstream of the catalytic converter and producing a second reading, a method for diagnosing at least one of the first or second readings or at least one of the first or second sensors, said method comprising inducing a special operating state of the combustion engine; and upon reaching the special operating state, monitoring the first reading and the second reading to determine if either the first reading or the second reading is less than a first threshold value within a first time span following said special operating state.
 2. The method according to claim 1, wherein no fuel is supplied to the combustion engine in the special operating state.
 3. The method according to claim 1, further comprising if both the first reading and the second reading is less than the first threshold value within the first time span, determining whether both the first reading and the second reading remains less than the first threshold value during a second time span.
 4. The method according to claim 3, further comprising incrementing or decrementing a counter if one of the first reading or the second reading exceeds the first threshold value within the second time span.
 5. The method according to claim 4, further comprising determining a counter threshold value for the status of the counter, and generating a signal if the counter is equal to the counter threshold value.
 6. The method according to claim 1, further comprising determining a steady state deviation between the first reading and the second reading, and adding the steady state deviation to one of the first reading and the second reading during the second time span.
 7. The method according to claim 1, further comprising diagnosing the catalytic converter unit based upon a diagnosis of the first reading and the second reading or at least one of the first or second sensors during the special operating state.
 8. A diagnosis device for use in a system comprising a combustion engine producing an exhaust gas, a catalytic converter for processing said exhaust gas, a first sensor arranged in the exhaust gas upstream of the catalytic converter and providing a first reading, and a second sensor arranged in the exhaust gas downstream of the catalytic converter and producing a second reading, said diagnosing device for diagnosing at least one of the first or second readings or at least one of the first or second sensors, comprising means for inducing a special operating state of the combustion engine; and means for monitoring the first reading and the second reading upon reaching the special operating state to determine if either the first reading or the second reading is less than a first threshold value within a first time span following said special operating state.
 9. The diagnosis device according to claim 8, comprising at least one processing unit having an input for receiving the first reading and the second reading, whereby the processing unit executes a program code.
 10. A computer program data transferable into a memory of a processing unit for operating a diagnosis device for use in a system comprising a combustion engine producing an exhaust gas, a catalytic converter for processing said exhaust gas, a first sensor arranged in the exhaust gas upstream of the catalytic converter and providing a first reading, and a second sensor arranged in the exhaust gas downstream of the catalytic converter and producing a second reading, said computer program comprising instructions performable by said processing unit for diagnosing at least one of the first or second readings or at least one of the first or second sensor, said computer program comprising instructions including instructions for inducing a special operating state of the combustion engine; and instructions for monitoring the first reading and the second reading upon reaching the special operating state to determine if either the first reading or the second reading is less than a first threshold value within a first time span following said special operating state. 